A fragmentation technology is introduced in the 802.11b protocol that supports low rate data transmission. The so-called fragmentation technology means that a MAC service data unit (MSDU) or a MAC management protocol data unit (MMPDU) is split into multiple segments at a MAC layer, and each segment is referred to as a fragment of the MSDU or the MMPDU. When receiving of a fragment fails, only the fragment that fails to be received is allowed to be retransmitted in the fragmentation technology, and there is no need to retransmit the entire MSDU or MMPDU, thereby improving robustness and a throughput rate of a network.
To support a higher data transmission rate, an A-MPDU technology is introduced in the 802.11n protocol. The A-MPDU technology means that an MSDU or an aggregate-MSDU (A-MSDU) is encapsulated to obtain a MAC protocol data unit (MPDU), and multiple MPDUs are transmitted in an aggregate manner. Aggregated MPDUs are distinguished using MPDU delimiters. In one A-MPDU, a maximum of 64 encapsulated MSDUs are allowed to aggregate for transmission. In the A-MPDU technology, a receiving state of an A-MPDU is fed back using a block acknowledgement (BA) frame. Each bit in a BA bitmap included in the BA frame correspondingly indicates a receiving state of one MSDU or A-MSDU. A first bit in the BA bitmap correspondingly indicates a receiving state of an MSDU with a first sequence number (SN) in a BA starting sequence control field, and so on.
In the 802.11ax protocol in a new generation wireless local area network (WLAN) system, an orthogonal frequency division multiple access (OFDMA) technology is introduced. Multiple users who use the OFDMA technology transmit data on different subchannels. As shown in FIG. 1, multiple stations (STAs) transmit A-MPDUs on corresponding subchannels. Each MPDU in the A-MPDUs is obtained by encapsulating one MSDU or one A-MSDU, and cannot be obtained by encapsulating a fragment of an MSDU. The multiple STAs use pad bits to enable data transmitted by the multiple STAs to align in terms of time. Then, a resource occupied by the pad bit may be used to transmit a fragment of an MSDU or an A-MSDU in order to improve transmission efficiency. Therefore, the new generation WLAN system needs to support both the MSDU fragmentation technology and the A-MPDU technology. That is, an MPDU in an A-MPDU may be obtained by encapsulating an MSDU or an A-MSDU, or may be obtained by encapsulating an MSDU fragment or an A-MSDU fragment. For ease of description, an MSDU fragment in the following description may also be understood as an A-MSDU fragment.
Because an existing compressed BA frame can only indicate a receiving state of an A-MPDU including MPDUs obtained by encapsulating only MSDUs, and cannot indicate a receiving state of an A-MPDU including an MPDU that is obtained by encapsulating an MSDU fragment in the new generation WLAN system. Currently, a receiving state of an A-MPDU including an MPDU obtained by encapsulating an MSDU fragment in the new generation WLAN system can be indicated using the following two solutions. However, there are some disadvantages, and details are as follows.
Solution 1: A Manner of Allocating an SN of an MSDU is Changed.
A unique SN is allocated to an MSDU or an MSDU fragment. In this way, different fragments of a same MSDU no longer have a same SN, and have respective SNs. Two bits in a sequence control field of an MPDU are used to indicate MSDU fragments that belong to a same MSDU. Each bit in a compressed BA frame is used to indicate a receiving state of an MSDU or an MSDU fragment corresponding to one SN. Therefore, a receive end indicates a receiving state of an A-MPDU by sending a compressed BA frame to a transmit end. In solution 1, an existing manner of allocating an SN of an MSDU is changed, and the solution is relatively complex. Once an indication of bits used to indicate MSDU fragments that belong to a same MSDU is incorrect, a receiver incorrectly considers that fragments coming from multiple MSDUs belong to a same MSDU.
Solution 2: A Quantity of Fragments of a Same MSDU is Restricted in an A-MPDU.
It is stipulated that one A-MPDU may include fragments of multiple MSDUs, but one A-MPDU can include only one fragment of a same MSDU. In this way, each bit in a compressed BA frame can indicate a receiving state of an MSDU or an MSDU fragment according to an SN sequence. Therefore, a receive end indicates a receiving state of an A-MPDU by sending a compressed BA frame to a transmit end. Therefore, in solution 2, multiple fragments of a same MSDU are not allowed to aggregate in one A-MPDU for transmission.
In conclusion, a receiving state of an A-MPDU including an MSDU fragment cannot be well indicated using existing solutions, and the existing solutions have some disadvantages.